Problem in mouse embryonic stem cell culture

Hello everybody
Iam culturing R1 strain of mouse ES cells. The problem iam facing is that the cells die after 2 passages. The culture conditions are the same as given in the protocol- 15%FBS, 1x beta mercapto ethanol(final concentration), 1x NEAA(final concentration) 1x Lif(final concentration), and the volume was made up with DMEM(lonza). For the feeders the MEFs were treated with mitomycin-c for 2 hrs. Washed thrice with 1x PBS and trypsinized for 30 sec and plated 20,000 cells/8cmsq.
Can you help me out with the problem.
Regards,
Dhanur. P. Iyer